Wordpress RSS feed modification

I need to have the default Wordpress RSS feed modified to suit my needs.

The RSS feed needs to be dynamic, it needs to be able to have a feature where is a user calls on the following URL:

[url removed, login to view];req=1&amp;or=4,5,6

It will show entries from that are in category 1, but are also in either category 4,5 or 6 at the same time. Hence the post has to be in at least 2 categories to appear in the feed but it is required to be in category 1 while 4,5 and 6 are acceptable second categories.

This is an extension of the default category feed feature in Wordpress. In plain english, I want to enable my users to be able to see a feed that shows the entries in the Car category but the entries that are shown have to also be in either the BMW, Audi or Lexus category in addition to the Car category.